Clay is a plastic material consisting of the very fine particles resulting from the weathering of rocks. While it is true that large clay beds have formed in the manner sulis describes, the presence of running water is only necessary to account for the transport of clay from one place to another. The actual formation of clay (i.e.) the formation of the tiny particles is due to the combined effects on soft rock of heat, cold, rain and wind, assisted in a great many cases by the grinding action of the advancing and retreating glaciers of the several ice ages which affected this country. In some cases it was washed elsewhere as sulis says, but in vast areas it simply remained in situ, mingled with the larger particles and became clay soil (i.e. 50% or more of clay particles, but deficient in humus and lime), or gradually acquired a covering of humus forming litter and some lime and other detritus and became loam soil(i.e.20% - 40% of clay particles) .
Clay itself is lacking in either lime or humus, and is therefore naturally acid.
However, clay should not be confused with marl, which is the decomposed paste residue of limestone rock and is therefore alkaline, although some soils contain both marl and clay, and are known as Argillaceous marls.
The deposit of fossils which Gardener refers to took place, of course, before this island was thrust upwards from the sea; the formation of clay did not start to take place until long afterwards.
